1. Field of the Invention
The present invention relates to a method of and an apparatus for dressing a polishing cloth, and more particularly to a method of and an apparatus for dressing a polishing cloth which is used to polish a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithograpy. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the abrasive cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotating.
After the workpiece has been polished and before another workpiece is polished, the polishing cloth is processed to recover its original polishing capability. Various processes have been and are being developed for restoring the polishing cloth, and are collectively called "dressing". The polishing cloth is dressed in order to enable the polishing apparatus to perform a good polishing function at all times without undesired degradation of a polishing performance which would otherwise be caused if the polishing cloth were not dressed, but rather used continuously. If the polishing apparatus suffers degradation of polishing performance, then it may not be able to polish a workpiece at a constant polishing rate or to a flat finish, or may cause the polishing cloth to have an unduly short service life.
Since one of purposes for dressing the polishing cloth is to remove a remaining abrasive liquid on the polishing cloth, ground-off chips of the workpiece, and worn pieces of the polishing cloth, it is often dressed while water is being supplied to the polishing cloth for thereby discharging the remaining abrasive liquid, the chips, and the worn pieces from the polishing cloth.
After the polishing cloth has been dressed with water supplied thereto, a new workpiece to be polished is set in place for a new polishing operation. However, the new workpiece cannot be polished at a sufficiently high polishing rate in an initial polishing period because an abrasive liquid is not present on the polishing cloth before the new workpiece starts being polished or the abrasive liquid is diluted with the water that has been supplied in the preceding dressing process and remained on the polishing cloth. It takes a certain period of time until the abrasive liquid is fully distributed over and impregnated in the polishing cloth or the concentration of the abrasive liquid on the polishing cloth increases to a predetermined level.
Since the new workpiece cannot be polished at a sufficiently high polishing rate in the initial polishing period, the total period of time required in each polishing process is relatively long, and hence the processing capability of the polishing apparatus per unit time is relatively low.
Furthermore, inasmuch as the abrasive liquid is not uniformly distributed over and impregnated in the polishing cloth until the abrasive liquid is fully spread over the polishing cloth, the performance of the polishing apparatus tends not to be uniform. Thus, the flatness of the polished workpiece tends to fail to reach a desired level.